Long-range Ni/Mn structural order in epitaxial double perovskite La2NiMnO6 thin films

We report and compare the structural, magnetic, and optical properties of ordered La2NiMnO6 thin films and its disordered LaNi0.5Mn0.5O3 counterpart. An x-ray diffraction study reveals that the B-site Ni/Mn ordering induces additional XRD reflections as the crystal symmetry is transformed from a pseudocubic perovskite unit cell in the disordered phase to a monoclinic form with larger lattice parameters for the ordered phase. Polarized Raman spectroscopy studies reveal that the ordered samples are characterized by additional phonon excitations that are absent in the disordered phase. The appearance of these additional phonon excitations is interpreted as the clearest signature of Brillouin zone folding as a result of the long-range Ni/Mn ordering in La2NiMnO6. Both ordered and disordered materials display a single ferromagnetic-to-paramagnetic transition. The ordered films display also a saturation magnetization close to 4.8 mB/f.u. and a transition temperature (FM-TC) around 270 K, while the disordered ones have only a 3.7 mB/f.u. saturation magnetization and a FM-TC around 138 K. The differences in their magnetic behaviours are understood based on the distinct local electronic configurations of their Ni/Mn cations.Comment: 15 pages, 5 fig

Phase formation, phonon behavior, and magnetic properties of novel ferromagnetic La3BAlMnO9 (B = Co or Ni) triple perovskites

In the quest for novel magnetoelectric materials, we have grown, stabilized and explored the properties of La3BAlMnO9 (B = Co or Mn) thin films. In this paper, we report the influence of the growth parameters that promote B/Al/Mn ordering in the pseudo-cubic unit cell and their likely influence on the magnetic and multiferroic properties. The temperature dependence of the magnetization shows that La3CoAlMnO9 is ferromagnetic up to 190 K while La3NiAlMnO9 shows a TC of 130 K. The behavior of these films are compared and contrasted with related La2BMnO6 double perovskites. It is observed that the insertion of AlO6 octahedra between CoO6 and MnO6 suppresses significantly the strength of the superexchange interaction, spin-phonon and spin-polar coupling.Comment: 13 pages, 3 fig

Enhanced Raman Microprobe Imaging of Single-Wall Carbon Nanotubes

We explore Raman microprobe capabilities to visualize single wall carbon nanotubes (SWCNTs). Although this technique is limited to a micron scale, we demonstrate that images of individual SWCNTs, bundles or their agglomerates can be generated by mapping Raman active elementary excitations. We measured the Raman response from carbon vibrations in SWCNTs excited by confocal scanning of a focused laser beam. Carbon vibrations reveal key characteristics of SWCNTs as nanotube diameter distribution (radial breathing modes, RBM, 100-300 cm(exp -1)), presence of defects and functional groups (D-mode, 1300-1350 cm(exp -1)), strain and oxidation states of SWCNTs, as well as metallic or semiconducting character of the tubes encoded in the lineshape of the G-modes at 1520-1600 cm(exp - 1). In addition, SWCNTs are highly anisotropic scatterers. The Raman response from a SWCNT is maximal for incident light polarization parallel to the tube axis and vanishing for perpendicular directions. We show that the SWCNT bundle shape or direction can be determined, with some limitations, from a set of Raman images taken at two orthogonal directions of the incident light polarization

Multiferroic double perovskites: Opportunities, Issues and Challenges

Despite the great technological and scientific importance, a well-controlled coupled behavior in magnetoelectric multiferroic materials at room temperature remains challenging. We demonstrate that the self-ordered A2BBO6 (e.g., A = La, B = Mn, and B = Ni) double perovskites provide a unique alternative opportunity to control and/or to induce such behavior in oxides. In this paper, we outline and discuss the various challenges and bottleneck issues related to this class of materials using La2NiMnO6 as a prototype. Details of the synthesis strategies, using pulsed laser deposition as a central tool, to obtain a meticulous control over cation ordering is illustrated. The possible impact of Ni/Mn cation ordering on the magnetic ground states, ferromagnetic transition temperature, phonon behavior, and spin-phonon coupling are presented. Our results are also compared with the current literature on related compounds.Comment: 12 pages, 3 figure

Magnetic Ground State of Pr0.89_{0.89}LaCe0.11_{0.11}CuO4+α−δ_{4+\alpha-\delta} with Varied Oxygen Depletion Probed by Muon Spin Relaxation

The magnetic ground state of an electron-doped cuprate superconductor Pr$_{1-x}$LaCe$_x$CuO$_{4+\alpha-\delta}$ ($x=0.11, \alpha\simeq0.04$) has been studied by means of muon spin rotation/relaxation (\msr) over a wide variety of oxygen depletion, $0.03\le\delta\le0.12$. Appearance of weak random magnetism over entire crystal volume has been revealed by a slow exponential relaxation. The absence of $\delta$-dependence for the random magnetism and the multiplet pattern of muon Knight shift at higher fields strongly suggest that the random moments are associated with excited Pr$^{3+}$ ions under crystal electric field.Comment: 6 pages, 4 figures, submitted to J. Phys. Soc. Jp

Infrared optical properties of Pr2CuO4

The ab-plane reflectance of a Pr2CuO4 single crystal has been measured over a wide frequency range at a variety of temperatures, and the optical properties determined from a Kramers-Kronig analysis. Above ~ 250 K, the low frequency conductivity increases quickly with temperature; the resistivity follows the form e^(E_a/k_BT), where E_a ~ 0.17 eV is much less than the inferred optical gap of ~ 1.2 eV. Transport measurements show that at low temperature the resistivity deviates from activated behavior and follows the form e^[(T_0/T)^1/4], indicating that the dc transport in this material is due to variable-range hopping between localized states in the gap. The four infrared-active Eu modes dominate the infrared optical properties. Below ~ 200 K, a striking new feature appears near the low-frequency Eu mode, and there is additional new fine structure at high frequency. A normal coordinate analysis has been performed and the detailed nature of the zone-center vibrations determined. Only the low-frequency Eu mode has a significant Pr-Cu interaction. Several possible mechanisms related to the antiferromagnetism in this material are proposed to explain the sudden appearance of this and other new spectral features at low temperature.Comment: 11 pages, 7 embedded EPS figures, REVTeX

Characterization of fecal nitrogen forms produced by a sheep fed with 15N labeled ryegrass

Little is known about nitrogen (N) forms in ruminant feces, although this information is important to understand N dynamics in agro-ecosystems. We fed 15N labeled ryegrass hay to a sheep and collected 15N labeled feces. Nitrogen forms in the feces were characterized by chemical extractions, solid-state cross polarization 15N nuclear magnetic resonance spectroscopy (SS CP/MAS 15N NMR) and Curie-point pyrolysis-gas chromatography/mass spectrometry (Cp Py-GC/MS). A 4months incubation experiment was conducted to assess N release from the feces. Half of the fecal N could be ascribed to bacterial and endogenous debris and a third to undigested dietary N. About a tenth of the fecal N was mineralized during the incubation experiment. The 15N abundance of nitrate released during the incubation remained constant and close to the 15N abundance of the total feces N. The NMR analysis of the feces showed that most of the N was present in proteins, while some was present as heterocyclic N, amino acids and ammonium. The Cp Py-GC/MS analysis confirmed the presence of proteins, amino acids and heterocyclic N in the feces. Comparing these results to those obtained from the 15N labeled hay suggests that some N compounds present in the plant were not digested by the animal, and that the animal excreted de novo synthesized N compounds. The low content in ammonium and amino acids, the low rate of N release from these feces during the incubation and the relatively high fecal protein content, particularly the hard to mineralize undigested and microbially bound forms, can explain the low transfer of N from these feces to crops observed in a previous wor

Bestimmung von Gärrestzusammensetzungen zum gezielten Substratmanagement von Böden

Mit dem weltweit wachsenden Energiebedarf gewinnen erneuerbare Energiequellen rapide zunehmend an Bedeutung. Eine zukunftsträchtige Energiequelle ist die Vergärung von Biomasse in Biogasanlagen. Dabei fallen Gärreste an, die u. a. zur organischen Düngung von Böden bzw. als Humusersatz eingesetzt werden könnten; die Wirksamkeit hängt jedoch von der noch weitgehend unbekannten Zusammensetzung der organischen Substanzen ab. Hinsichtlich des Einflusses von Gärresten auf den Boden-Stickstoff- und -Humushaushalt wurden diverse pflanzliche Ausgangsmaterialien und Gärreste als Vollgärrest, abgetrenntes Fugat und Feststoff mit Curie-Punkt Pyrolyse-Gaschromatographie-Massenspektrometrie (Py-GC/MS) und Pyrolyse-Feldionisation Massenspektrometrie (Py-FIMS) auf der molekularen Ebene untersucht. Bei den zugeordneten Py-GC/MS-Signale dominierten Markersubstanzen der Kohlenhydrate, Phenolderivate, aliphatische Lipide, Alkylaromaten und weitere cyclische Verbindungen. Die Zuordnung von N-Verbindungen erfolgte anhand von 15N-markierten Proben, bei denen Derivate von Pyrazol, Pyrrol, Pyridin, Indol, Pyrimidin und aromatische Nitrile vorherrschten. Die Zuordnung der Py-FIMS-Indikatorsignale zu relevanten biogenen Substanzklassen ergab in allen Gärresten bis zu 33 % Sterole mit den dominierenden Verbindungen ß-Sitosterin (m/z 414) bzw. Stigmastanol (m/z 416). Dagegen wurden im Feststoff deutlich höhere Anteile von Kohlehydraten (m/z 61, 117, 126), Ligninen (m/z 208, 210) und Fettsäuren (m/z 242, 280) detektiert. Die Thermogramme und Massenspektren zeigten eine ähnliche chemische Zusammensetzung der Vollgärreste und Fugate, die wesentlich durch die vergorenen Ausgangsmaterialien bestimmt wird. Zukünftig werden Untersuchungen der korrespondierenden Bodenproben aus Topfversuchen Aussagen über den Einfluss der Gärreste auf den Humusgehalt liefern und 15N-Anreicherungen den Verbleib von gärrestbürtigen N-Verbindungen in Böden aufklären